MSMechanical SciencesMSMech. Sci.2191-916XCopernicus PublicationsGöttingen, Germany10.5194/ms-7-119-2016Mechanical design, analysis and testing of a large-range compliant microgripperLiuYilinXuQingsongqsxu@umac.mohttps://orcid.org/0000-0002-1700-322XDepartment of Electromechanical Engineering, Faculty of Science and Technology,
University of Macau, Avenida da Universidade, Taipa, Macau, ChinaQingsong Xu (qsxu@umac.mo)12April20167111912615December20159March201624March2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://ms.copernicus.org/articles/7/119/2016/ms-7-119-2016.htmlThe full text article is available as a PDF file from https://ms.copernicus.org/articles/7/119/2016/ms-7-119-2016.pdf
This paper presents the mechanical design, analysis, fabrication, and testing
procedures of a new large-range microgripper which is based on a flexible
hinge structure. The uniqueness of the gripper is that the gripper arms not
only provide large gripping range but also deliver approximately rectilinear
movement as the displacement in nonworking direction is extremely small. The
large gripping range is enabled by a mechanism design based on dual-stage
flexure amplifier to magnify the stroke of piezoelectric actuator. The
first-stage amplifier is a modified version of the Scott Russell (SR)
mechanism and the second-stage amplifier contains a parallel mechanism. The
displacement amplification ratio of the modified SR mechanism in the gripper
has been enlarged to 3.56 times of the conventional design. Analytical static
models of the gripper mechanism are developed and validated through
finite-element analysis (FEA) simulation. Results show that the gripping
range is over 720 µm with a resonant frequency of 70.7 Hz
and negligible displacement in nonworking direction. The total amplification
ratio of the input displacement is 16.13. Moreover, a prototype of the
gripper is developed by using aluminium 7075 for experimental testing.
Experimental results validate the analytical model and FEA simulation
results. The proposed microgripper can be employed in various microassembly
applications such as pick-and-place of optical fibre.
Introduction
With the rapid development in micro-electromechanical systems
(MEMS) and precision engineering, extensive research on automatic
micromanipulation and microassembly technology has been carried out. As one
of the usual manipulators, microgripper is an essential device in the
micro-/nanomanipulation . A
typical microgripper consists of the actuator and guiding mechanism.
Recently, researchers have created different kinds of the grippers, such as
the adhesion-type and mechanical-type grippers
. In some applications, the
gripper operating performance is limited by the gripping range. So, many
amplification devices, such as bridge-type amplifiers, have been used to
enhance the output displacement . These studies are of
practical significance for the realization of the microassembly technology.
To realize an ultrahigh precision displacement, the flexure mechanisms have
been widely applied
. In
addition, lots of micro-/nanogrippers have been driven using smart actuators,
e.g., piezoelectric stack actuators, which have been applied to biological
cell manipulation and MEMS
. Piezoelectric (PZT)
actuator has many advantages, such as high resolution, faster response speed,
larger force output and low power consumption
. But it has
limited output displacement. Hence, the output is not enough for the grasping
task in many cases. This issue can be overcome by resorting to displacement
amplification mechanisms.
Concerning the structure design of a gripper, various designs have been
devised by using compliant mechanisms. The output motion of the compliant
mechanisms is produced by using the elastic deformation of the material.
Hence, compliant mechanisms can provide some advantages, such as friction
free and lubrication free. However, the output motions of majority of
existing microgrippers are generated in a rotary way. Hence, if the gripper
tips come to contact with the target object which has a curved shape, the
reaction force will push out the object. Scott Russell (SR) mechanism has
been used to design micromanipulation systems previously
. SR mechanism has a
simple structure and it can provide rectilinear output motion. But its
disadvantage is also clearly. That is, its amplification ratio is not large
enough. This means that the SR mechanism may reduce the output displacement
of the actuator. However, the output of a PZT actuator in a micromanipulator
is small already. Hence, one motivation of this work is to overcome this
problem by devising an improved SR mechanism.
Motivated by the aforementioned review, this paper presents the mechanical
design, fabrication, and testing procedures of a new large-range compliant
microgripper, which is based on flexure-hinge structure. The structure of the
gripper is illustrated in Fig. 1. The main advantage of the gripper is that
the gripper arms not only provide large-range output displacement but also
produce approximately rectilinear movement, because the nonworking
directional displacement is extremely small. In this design, the gripper is
actuated by only one PZT actuator which is fixed in the middle of the device.
The actuator delivers the resolution of 0.45 nm (open-loop) and its
output stroke is 45 µm. A two-sided dual-stage flexure amplifier
is designed to magnify the stroke of PZT actuator twice. The first-stage
amplifier is a modified version of the SR mechanism, which is placed in a
hypotenuse position. The monolithic components are connected by using
circular notch hinges . With this SR mechanism, the
gripper can provide both amplified and approximately rectilinear output. The
amplification ratio of the modified SR mechanism has been enlarged to
3.56 times of the existing design. The second-stage amplifier is a
parallelogram which can resist the nonworking directional deformation. This
amplifier has been used to design a metamorphic gripper for microassembly in
the previous work .
The mechanical structure of the designed microgripper.
CAD model of guiding mechanism.
In this work, the total amplification ratio for the input displacement is 16
and the achieved gripping range is 720 µm. For experimental
verification, a mesoscale prototype is developed by using a PZT actuator. The
actuator has a length of 68 mm. The gripper size can be reduced by
changing the actuation approach. Detailed design procedures are presented in
the following parts of the paper.
Mechanism design and modeling
The mechanism design of the microgripper is presented in this section. It
involves the guiding mechanism design, the first-stage and second-stage
amplifier design, and dynamics modeling of the gripper.
Guiding mechanism design
For driving the gripper, the actuation displacement is guided by four leaf
flexures. The computer-aided design (CAD) model of the guiding mechanism is
depicted in Fig. 2. When the output force of the PZT actuator is exerted on
this guiding mechanism, the guiding flexures produce a purely translational
movement. Each leaf flexure behaviors like a fixed-guided beam. In view of
the boundary conditions of the two ends of the beam, i.e., the rotation angle
of the ends is zero and the deformation of the guided end is δx, the
following equations are generated.
0=Fl22EI-MlEIδx=Fl32EI-Ml2EI,
where l is the length of the leaf flexure, F denotes the force, M
represents the moment, and E is elastic modulus of the material.
The moment of inertia can be calculated as follows.
Ib=mbl212,
where mb is the mass of the leaf flexure.
The solutions to the above two Eqs. (1) and (2) are shown below.
M=Fl2δx=Fl312EI
From the kinetic energy equation and the kinematic relationship of the
mechanism, the expression of the equivalent mass can be derived:
M1=ma142+4mb182+4Ib1(8l)2,
where ma is the mass without the leaf flexure, as shown in Fig. 6
later.
The potential energy of the guiding flexures mechanism is obtained as follows.
V=412Ksθz2=2Ksq2(8l)2,
where Ks is the stiffness, θz is the rotational angle,
and q denotes the deformation at the end of each beam. The details can be
referred to the previous work .
For the presented structure, the stiffness Ks is obtained below
:
Ks=2γKθEIl,
where γ=0.85 and Kθ=2.6686.
Hence, the natural frequency of the guiding mechanism is calculated as shown
below.
f=KM1,
where
K=Ks(4l)2.
Schematic of Scott Russell mechanism.
First-stage amplifier design
The first-stage amplifier is a modified version of the SR mechanism. At
first, the model of the SR mechanism will be introduced. As is known, when
the lines AB = BC = BD =L, the structure will be consider as SR
mechanism, which is shown in Fig. 3.
In order to solve the relationship between the input at point A and the
output at point C, assume that the displacement ΔyA is
close to zero after input. Then, in the right triangle, one has the
relationship (11). By solving the trigonometric relationship, the results of
output displacement and the amplification ratio can be obtained as shown in
Eqs. (12) and (13):
ΔyA+lAD2+Δxc+lCD2≈lAD2+lCD2,ΔxC=lDC2-2LADΔyA-ΔyA212-lDC,R1≈-2L2-lAD212lAD=-tanα,
where L=38.84mm and lAD=72.75mm in this work.
From the above results of the amplification ratio (13), it is seen that the
output of SR mechanism is decreased when the length of segment CD is smaller
than that of AD. It means that tanα is always less than one. Hence,
although SR mechanism can provide a rectilinear output, the amplification
ratio is less than one. This is the disadvantage of the conventional SR
mechanism. The angle α is set as 20.47∘ in this work. With
this setting, the amplification ratio of the SR mechanism is calculated as
follows.
R1≈-(2L)2-lAD212lAD=-tan(20.47)=-0.37
The minus sign means the negative direction of the output displacement.
In the modified SR mechanism design, the ratio between AB and BC is
different. This setup not only improves the amplification ratio of the SR
mechanism, but also produces approximately rectilinear output. The new
amplification ratio can be calculated as follows:
R2≈lAB+lBCtanαlAB.
A CAD model of this modified amplifier and its main parameters are shown in
Fig. 4. The input from the PZT actuator is applied at the point A in the
y axis direction. The new amplification ratio can be calculated as follows:
R2≈22.01+55.89tan20.422.01=1.32,
where LAB=22.01mm and lBC=55.89mm.
The ration η between the amplification ratios of the new design and
conventional SR mechanisms is obtained as follows.
η=R2R1≈lAB+lBClAB
That is,
η=1.320.37≈(22.01+55.89)22.01=3.56,
which indicates that the amplification ratio has been enlarged by 3.56 times
with the new SR mechanism.
Modified version of Scott Russell mechanism.
Model of the second-stage amplifier.
Second-stage amplifier design
The second-stage amplifier is a parallelogram mechanism with leaf flexures.
After the first stage of amplification, the ratio is not large enough. So,
the main objective of this module as an essential device is to further
enlarge the output twice and to resist the nonworking directional
displacement. The structure of this module and its main parameters are
illustrated in Fig. 5. By combining the two amplifiers together, the
performance of this microgripper can be further improved. The final
amplification ratio of the gripper can be calculated by:
R3≈lAB+lBClIGtanαlABlyIE
That is,
R3≈22.01+55.89×83×tan(20.4)22.01×12.75=8.59,
which means that the amplification ratio R3 is 8.59 for one gripper arm.
During the gripping operation, the left and right arms of the gripper move
together. Considering that there is only one actuator in the gripper device,
the input has been amplified by 17.18 times.
Referring to Fig. 5, the rotational stiffness of point I can be calculated by
(Ai and Xu, 2014):
Kr=FylHG4+lFG2lIG212.
The input stiffness can be calculated as:
K=Kr+KrlIG2lHG2.
Moreover, the equivalent mass of the gripper is shown below:
M=M1+95m2+18125m3+128725m4+479740m5,
where M1 is calculated from Eq. (6) and the mass distribution of the
gripper is illustrated in Fig. 6.
Then, the natural frequency in unit of Hertz can be calculated by
f=12πKM.
Mass distribution of the gripper.
Finite-element analysis simulation
In order to verify the analytical models as developed in the previous
subsection, finite-element analysis (FEA) simulation is carried out with
ANSYS to test the performance of the gripper. The simulation study includes
the output displacement, nonworking directional displacement, and modal
analysis. The selected material is aluminium 7075, which is known to have
good mechanical properties like resistance to stress corrosion cracking and
high strength in low temperature. The properties of the material are listed
in Table 1. The simulation study is necessary to confirm the performance
before the manufacturing of the prototype.
As shown in Fig. 7a, the maximum output displacement of the gripper is
364.4 µm when the input displacement for the gripper is assigned
as 45 µm, which is the maximum output of the PZT actuator as used
in the developed prototype. In addition, it is derived from Fig. 7b that the
output displacement in the nonworking direction is only 61.2 nm,
which is negligible in comparison with the output displacement in the working
direction.
Properties of aluminium 7075 alloy.
ItemValueUnitsElongation11 %–Density2810kgm-3Modulus of elasticity72GPaYield strength455MPaTensile strength524MPaPoisson ratio0.33–
Moreover, modal analysis simulation has been conducted to examine the dynamic
performance of the gripper structure in terms of resonant mode shapes and
frequencies of the gripper structure. The simulation results of the first six
resonant mode shapes are shown in Fig. 8, and the corresponding resonant
frequencies are described in Table 2. It is seen that the first frequency of
the gripper is 70.7 Hz. A high natural frequency enables a quick
response of the gripper device. In addition, modes 3 and 4 are caused by the
vertical translations in the y axis, and modes 5 and 6 are induced by the
bending of the gripper arm around the z axis.
From the simulation results, it is observed that the objective of design has
been achieved. This microgripper can provide both a large-range output and an
approximately rectilinear movement. The rectilinear movement is useful for
gripping various micro-particles in practice.
FEA simulation results. (a) Maximum output of the gripper,
(b) displacement in nonworking direction.
The first six resonant mode shapes (a–f).
Experimental results
After the FEA simulation verification, the device has been manufactured.
Then, an experimental study is conducted to test the performance of the
prototype. The aim of the experiment is to verify the output displacement of
the gripper. The experimental setup is depicted in Fig. 9. A laser sensor is
used to detect the output displacement of the gripper tip. The sensor has a
resolution about 25 nm. A PZT actuator is used to actuate the gripper
and its specifications are listed in Table 3. Moreover, the actuator has a
working voltage range of 0–100 V.
Experimental setup of the microgripper.
It is observed that when the driving voltage approaches to 100 V, the
output displacement arrives at the largest value. The results collected by
the laser sensor are shown in Table 4. The open and close states of the
gripper tips are illustrated in Fig. 10. Moreover, for the purpose of
comparison, the analytical model result, FEA simulation result, and
experimental result are listed in Table 5. It is observed from Table 5 that
the experimental result is very close to the analytical and simulation
results, which verifies the effectiveness of the developed models.
The results of the conducted experimental studies verify the design of this
new large-range compliant microgripper. A modified version of SR mechanism is
proposed to enhance the performance of the original version. The objective is
to enlarge the amplification ratio and to resist the nonworking directional
displacement by using one more parallelogram module. The FEA simulation shows
that the nonworking directional displacement is 61.2 nm. This
performance can be optimized by an optimum design to be conducted in the
future work.
In order to obtain a larger gripping range in micromanipulation, the
dual-stage amplifier provides an amplification ratio of 8.066 per arm.
Totally, the gripping range is enlarged to be 16.13 times of the driving
displacement, which is enabled by the proposed new gripper design with
modified SR amplification mechanism. As compared with the existing grippers,
in which only one arm is actuated, the two-arm actuation approach with
dual-stage amplification mechanism produces a much larger output gripping
range.
For practical applications, the grasp force exerted on the object should lie
in an appropriate range . Hence, the grasp
force regulation is necessary to guarantee that an appropriate force is
applied to the grasped objects . In the
future work, the force sensing and control system module will be realized to
deliver a desired output force of the gripper for executing related
micromanipulation tasks. Pertinent application study will be carried out as
well.
Photos of the gripper tips before (a) and
after (b) the actuation.
Experimental results of output displacement of the compliant
microgripper.
ItemTest 1Test 2Test 3Test 4Output displacementof one gripper arm (µm)362363360363
Comparison of the results.
ItemTest 1Test 2Test 3Output displacementof one gripper arm (µm)362363360Error7.2 %0.88 %–Conclusions
In this paper, a new design of large-displacement gripper is
analyzed, fabricated, and tested. Each module of the gripper is designed in
detail. After the analytical model has been developed, simulation study has
been conducted to verify the performance of the design. Finally, a prototype
of the gripper has been manufactured for experimental testing. Results show
that the gripping range is 0–720 µm along with a large
displacement amplification of 16.13. It provides enough output motion for the
manipulation of majority of micro-particles. The amplification ratio of the
modified SR mechanism has been enlarged by 3.56 times relative to the
traditional design. In the future work, a precision motion and force control
module will be implemented on the developed gripper to produce a desired
output force dedicated to related micromanipulation tasks.
Acknowledgements
This work was supported in part by the Macao Science and Technology
Development Fund under Grant 090/2015/A3 and 052/2014/A1, and the Research
Committee of the University of Macau under Grant
MYRG078(Y1-L2)-FST13-XQS.
Edited by: K. Mianowski
Reviewed by: L. Bruzzone and M. Brandstötter
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